On-line Verification Method of Mass Flow Meter

Since mass flowmeters are sensitive to pressure fluctuations, the verification of mass flowmeters requires small pressure fluctuations during the verification process. The impact of fluid pressure pulsation on the measurement of mass flowmeters is described in detail in 8.2.3, 8.2.4 of ISO10790, and 8.2.2.3, 8.2.4(c) of the draft API standard. It can be seen from the previous description that the flow standard used in the mass flowmeter online verification is the standard volume tube. Among the two main standard volumetric tubes, the spherical volumetric volumetric volumetric volumetric tube has complex structure, high pressure loss, large fluid pressure fluctuations, and high price, making it difficult to meet the needs of mass flowmeter verification. In contrast, the piston type volumetric tube has many advantages such as light and small, simple structure, low cost, fast flowmeter verification, and easy to move. It is undoubtedly the best choice for online real liquid calibration of mass flowmeters. 1. Understand the piston type volumetric tube The piston type volumetric tube is different from the traditional pipeline type volumetric tube. It does not use its large enough standard volume to ensure the extremely high pulse measurement accuracy, but uses a piston type structure, double clock insertion method, precision A new implementation of advanced technologies such as photoelectric detection. This precise and compact standard volume tube has more and more important advantages than traditional volume tubes: short verification time of flowmeter, small pulsation, small influence on fluid, small pressure loss, small volume, etc. The piston type volumetric tube consists of a smooth and hollow cylindrical flow tube, a piston with a poppet valve, a pair of photoelectric switches and a piston lifting mechanism. There is a double-layer sealing ring with a very low friction coefficient between the piston and the polished flow tube, and the metering piston combination is in full contact with the flow tube wall at all times. When the volume tube is running, the piston lift mechanism pulls it to the upstream end of the volume tube, and the lift valve on the piston is always open during the entire lifting process of the piston. After the lifting is completed, the lifting mechanism is disengaged from the piston. Under the joint action of the spring of the poppet valve, the force of the fluid, and the Venturi effect of the fluid passing through the poppet valve, the poppet valve closes smoothly and quickly, and then enters the calibration process. The two photoelectric detection switches on the volume tube record the start time, stop time and elapsed time of the piston passing through the standard volume section. According to the double clock insertion method and related standards, various measured parameters and data are calculated to complete Flow meter verification process. Among the existing piston-type volumetric tubes, the S series volumetric tubes of American Gary Brown Company are the best products. It adopts advanced designs such as free piston structure and chain-driven piston return system, which makes the piston-type volumetric tube extremely perfect. Simple and maintenance free. The free and passive piston movement of this series of volumetric tubes (no power boost required) reduces the pressure drop of the volumetric tube to the minimum (such as the S25 type volumetric tube when the medium is water and the maximum flow rate is 567m3/h. The pressure drop is only 52kPa), the pressure fluctuation generated during the calibration process is also extremely small, and the turndown ratio is very large (typically 1200:1), which is verified by various flowmeters. 2. Verification method of piston volumetric tube with online density meter The verification method of mass flowmeter with piston type volumetric tube with online density meter is an indirect quality verification method.“API standard draft”It is called inductive quality verification, and the specific configuration is shown in the figure above. The calibration system with this configuration is the basic solution for on-line calibration of mass flowmeters. This method has many expressions in the CMF standard text, and its advantage is simplicity. However, since the mass flowmeter is more sensitive to the pulsation of the fluid, the piston type volumetric tube always has a certain disturbance effect on the fluid during the verification operation, and the degree of its influence increases with the increase of the flow rate. Therefore, the piston type volumetric tube is used for The maximum flow value of the mass flow meter is generally lower than the maximum flow value of the non-mass flow meter. The products of some manufacturers may vary greatly, which requires the piston type volumetric tube used to use a product with a larger specification. Since there is no corresponding standard clause or test basis, what kind of disturbance and quantitative value can have a non-negligible impact on the mass flowmeter is still unknown. Of course, the requirements for the piston volume tube cannot be specified. The on-line mass flowmeter calibration system using this configuration is only determined by the volume tube manufacturer's own test or experience, which contains many uncertainties and is very unscientific. the

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